This invention relates to rotary machines and, specifically, to minimizing leakage flow between stationary and rotating parts of a packing assembly in a steam turbine.
Packing rings are used in steam turbines as a means of minimizing leakage flows between fixed and rotating steam path parts. These packing rings consist of arcuate ring segments, each of which has multiple axially spaced teeth that are either machined integrally with, or inserted into the arcuate ring segments. The segments together provide an annular seal. The packing ring segments are installed in grooves machined in mating stationary parts of the turbine such as diaphragms or casings, and are radially positioned to provide a close fit with a rotating part of the turbine, e.g., the rotor. Typically, in the packing seal area, the rotor is machined to provide axially spaced, annular lands that, along with the rotor surfaces between the lands, are regarded as part of the packing assembly. The close fit between the teeth and the rotor is called a xe2x80x9cclearance,xe2x80x9d and minimizing this clearance is essential in maximizing steam turbine efficiency.
Operational transient conditions of steam turbines during startups, shutdowns or load swings will often cause the rotating part to contact (i.e., xe2x80x9crubxe2x80x9d) the stationary part and wear the packing teeth to a larger clearance. This xe2x80x9crub,xe2x80x9d or wear, produces a mushroom shaped tooth profile, which will allow more leakage than a sharp tooth at an equivalent clearance. Normal turbine operation will also wear the packing teeth to a larger clearance. The amount of wear in a packing ring that is typical for a steam turbine can be described statistically for different steam turbine configurations based on field inspection data obtained when the units undergo maintenance. This data has been accumulated and has been analyzed to determine the Means and Standard Deviations of the wear data broken down by turbine stage number and code type.
Normal steam turbine design practice is to design the packing ring hardware to provide a tight, constant initial clearance and allow the clearances to open up during a startup by xe2x80x9crubbing inxe2x80x9d or wearing eventually over time, as described above. An important design parameter of the packing ring is that all of the packing teeth are machined to provide the same clearance on every tooth within an individual packing ring. Thus, when the packing ring wears, all of the teeth clearances within the packing ring increase to approximately the same amount. This is consistent with the data taken from the field.
This invention comprises apparatus and a method for minimizing leakage flow between the stationary and rotating parts of a rotary machine. Specifically, a packing seal assembly in a steam turbine is designed to have radial clearances that vary along the axial extent of the seal. This approach minimizes the leakage flow over the life of the seal by accounting for operational transients and expected seal wear. The invention described herein is described in connection with a labyrinth type seal but this invention could also be applied to other seal configurations. For example, this invention is applicable to both integral and inserted teeth packing ring designs of fixed or variable clearance, as well as packing rings that contain brush seals. This invention is also applicable to spill strip bucket tip seals of different designs.
The optimization of the seal is accomplished by either (a) machining individual teeth within a packing ring to different heights, and/or (b) machining the rotor lands to different heights. Either or both of these alternatives will result in non-constant radial clearances between the stationary (i.e., packing ring) and rotating (i.e., rotor lands) parts. Thus, as the turbine rotor wears away any tooth due to transient operation (i.e., xe2x80x9crubxe2x80x9d), there will still be sharp teeth remaining within the seal assembly. The location and number of teeth at a given clearance can be optimized to provide the minimal leakage for the expected operational conditions of the machine. This can be accomplished by developing a transfer function between the packing ring flow coefficient (i.e., leakage rate) and the expected wear of a given machine configuration. The optimized design would be statistically based because the wear data follows a statistically defined distribution as described further below.
Accordingly, in its broader aspects, the invention relates to a packing ring seal for use in a steam turbine for minimizing leakage flow between stationary and rotating steam path parts comprising a stationary annular seal ring having a plurality of axially spaced, annular sealing teeth extending in a radial inward direction and adapted to cooperate with sealing surfaces on a rotating component of the steam turbine, wherein radial clearance between the sealing teeth and the sealing surfaces varies by predetermined amounts.
In another aspect, the invention relates to a method of minimizing flow leakage axially along a rotatable machine shaft, between a stationary component surrounding the machine shaft and a plurality of surfaces rotatable with the machine shaft, the method comprising the steps of:
a) providing a labyrinth seal ring having a plurality of axially spaced, annular teeth;
b) mounting said seal ring above the machine shaft such that the plurality of axially spaced teeth cooperate with surfaces rotatable with mid machine shaft; and
c) establishing predetermined varying radial clearances between the axially spaced teeth and the surfaces rotatable with the machine shaft.